In the books, he never gets back with Mildrith. She joins a convent. Iseult loses her virginity immediately upon Uhtred capturing her, but she does die at Ethandun. He then goes off with Hild, the nun, but they are more friends who shag, because he is angry with Alfred, who doesn't reward him very well for his services in the swamp. Hild had been taken by the Danes when Cippenhamm fell and whored out to many. It was actually Alfred who saved her. He compelled Uhtred to help him, though.
Leofric also dies at Ethandun. Uhtred really liked Leofric.
In the first book, both Brida and Uhtred witness Ragnar's sister, Thyra, being captured by Kjartan and Sven the One Eyed. Sven is one eyed because, as children, he captured Thyra, strips her half naked, ties her to a tree and then tells her to touch his dong. Uhtred picks up the sword Sven had and screams, 'I will touch it!' and, after cutting Sven, who runs away with his friends, he cuts Thyra loose. They all go back to Ragnar the Elder's (in the book Ragnar the Elder has another son, Rorik, who is close to Uhtred in age but dies young) where they are separated to tell their stories, which chime up. Ragnar the Elder goes to Kjartan and, because Sven stripped Thyra only half naked, he is given half the punishment and thus loses only one eye. Kjartan was Ragnar the Elder's liege.
Once Ragnar the Elder is dead, Uhtred's life is in peril from Kjartan, who wants him dead as he is a witness to the hall burning, and Aelfric, Uhtred's paternal uncle. Alfred's protection preserves his life.
Other than Mildrith, Uhtred doesn't mistreat his women. He and Brida were very young when they started becoming intimate, like 14 and 13. Brida loses the baby, barely formed but old enough to tell it's a boy, and Ragnar's wife guesses what happened and tells Ragnar it's time she was married, so she's probably 13.
He's very, very young at the end of the first book, only 20 and not much older at the end of the second one, 21.